Stiffened composite structures are structures that are constructed of composite materials, such as fiber-reinforced composite materials, and typically include some form of structural frame that carries a skin. Some modern aircraft fuselages are examples of stiffened composite structures that include a skin operatively coupled to frame members that extend circumferentially around and that are spaced longitudinally along the inside of the fuselage and stringers that extend longitudinally along and that are spaced circumferentially around the inside of the fuselage. Typically, stiffened composite fuselages are constructed utilizing an inner mold line layup mandrel that includes stringer cavities, or forms, that extend longitudinally along the mandrel. Working from the top of the inner mold line layup mandrel, stringers are hand positioned in the stringer cavities.
Typical stringers, such as hat-shaped stringers, define cavities themselves, with these cavities needing to be filled with a structure that will result in a flush surface to which the skin may be applied. These filler structures often are referred to as stringer bladders, or simply bladders. In addition to the bladders, radius fillers, or noodles, are used at the interfaces between the outer most edges of the bladders and the stringers to ensure a smooth transition between the stringers and the skin, once it is applied (e.g., to ensure a smoother surface between the stringer and bladder, so that the skin isn't stressed by sharp transitions at the interface of the bladder and stringer). Following their placement, the bladders and noodles are vacuum-compacted in batches so that the bladders and noodles remain in place for subsequent application of the skin.
The vacuum compacting process utilizes a gas-impermeable flexible sheet of material that extends across the outer surface of the inner mold line layup mandrel and that is sealed to the inner mold line layup mandrel around the outer perimeter of the stringers and associated bladders and noodles that are being compacted. A vacuum is then applied between the sheet of material and the inner mold line layup mandrel to compress the bladders into the stringers. This process is referred to in the aerospace industry as “bagging.”
U.S. patent application Ser. No. 13/733,036, filed on Jan. 2, 2013 and entitled “SYSTEMS AND METHOD FOR ASSEMBLING STIFFENED COMPOSITE STRUCTURES,” U.S. patent application Ser. No. 14/012,911 filed Aug. 28, 2013 and entitled SYSTEMS AND METHODS FOR ASSEMBLING STIFFENED COMPOSITE STRUCTURES, and U.S. patent application Ser. No. 13/732,961 filed Jan. 2, 2013 and entitled SYSTEMS AND METHODS FOR ASSEMBLING STIFFENED COMPOSITE STRUCTURES disclose related systems and methods of assembling stiffened composite structures, and the complete disclosures of these applications are hereby incorporated by reference in their entirety for all purposes. As described in these applications, a stringer is often loaded onto the mold line layup mandrel first, often by hand, and a separate tool or step may be utilized to install the filler structure and radius fillers within the stringer cavity.
The loading of the stringers and the installation of the bladders and noodles, as well as the bagging process and the compacting process, are performed on the upper side of the inner mold line layup mandrel, which, for a fuselage of a commercial aircraft, can be very large. Stringers may be very long (greater than 20 feet long), and may be difficult to handle and move in their entirety. Moreover, these processes are all labor and time intensive. Further, often times a fuselage section may include two or more different types of stringers, where each type of stringer requires its own customized tooling. Such tooling is also very large, which may require excessive space for use and storage.